Abstract

The metastable $L1_2-Al_3Zr$ phase has been obtained as a solidification product on melt-spinning ternary Al-X-Zr (X = Cu, Ni) alloys. Different non-equilibrium effects of the metastable $Al_3Zr$ phase $(L1_2)$ have been observed in the as-solidified and heat-treated alloys. The solidification sequence begins with the formation of the $L1_2-Al_3Zr$ (cubic) phase as a primary phase, followed by heterogeneous nucleation of \alpha A1. Morphological changes in the primary phase result in a shape transformation from a faceted cube to one with concave interfaces and protrusions along the corners, having a preferential growth along the <111> direction. This is brought about by a kinetic effect taking place during the growth of the $L1_2-Al_3Zr$ phase into the surrounding liquid, as the alloy is quenched. In another instance, the primary $L1_2-Al_3Zr$ phase nucleates as solid-state precipitates of the same $L1_2-Al_3Zr$ phase on annealing, by dissolution and reprecipitation of solute, under the influence of moving grain boundaries. A third case shows the metastable $L1_2$ phase nucleating on the equilibrium $DO_2_3-Al_3Zr$ phase, upon solidification. This is attributed to the sluggish growth kinetics of the latter.

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Journal Article

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